Add rudimentary CONTRIBUTING.md

[akbad]

Add a basic CONTRIBUTING.md file based on useful knowledge I discovered about the codebase.

Sections included

• Environment setup (including link to install the exact protoc compiler version needed)
• Description of why and how to regen Python stub files for protobuf
• Architecture section with basic explanations of key classes/types

[kokarare1212]

Thank you for your PR!